1. Field of the Invention
This invention relates to a class of novel organic optical dyes of phthalocyanine derivatives and their applications in optical recording media, primarily to the use in recordable compact disc (CD-R).
2. Description of the Prior Art
Organic dyes have been widely employed in the field of optical recording of information. These recording media, which can only be recorded once but repeatedly played back, are therefore abbreviated as “WORM” (write once read many). Recordable compact discs, or the so-called CD-R, as the first example in disc format utilizing this technology, are known from “Optical Data Storage 1989,” Technical Digest Series, vol. 1, 45 (1989).
Among all the organic dyes for optical recording media, phthalocyanine derivatives are one of the most important categories, due largely to its high absorption in the near IR range (700˜900 nm). Compared to other organic dyes such as cyanines, phthalocyanine dyes exhibit better light-fastness and resistance to temperature and humidity.
Earlier literatures such as JP-A 154888 (1986), 197280 (1986), 246091 (1986), U.S. Pat. No. 4769307 (1987) and JP-A 39388 (1988) described phthalocyanines as a component material in the recording layer of an optical recording medium. However, in terms of sensitivity, solubility, reflectivity, recording performance and other related properties, the above-described phthalocyanines could not be considered as appropriate materials for an optical recording medium.
In order to improve the aforementioned disadvantages associated with the use of phthalocyanine as an optical recording material, JP-A 62878 (1991) provided phthalocyanines with bulkier (greater steric hindrance) substituents on its phenyl rings. These materials, however, did not meet the recording requirements. In U.S. Pat. No. 5,229,507 (1993), phenyl-substituted phthalocyanines (also called naphthalocyanines) were proposed but the dyes exhibited insufficient solubility. Under certain process conditions, dyes would precipitate in the course of spin coating.
Solubility issue was further addressed in U.S. Pat. No. 5,641,879 (1997) by introducing various bulkier substituents onto the phenyl rings of phthalocyanine. However, inadequate reflective index was found. Isomer effects on solubility were studied in U.S. Pat. No. 5,663,326 (1997). It was reported that composition of the two isomers having one pair of alkoxy substituents heading toward each other needed to be greater than 80% in order to obtain desired solubility. It is obviously tedious for dye manufacturing processes and seemingly impractical to assure the isomer composition for quality control.
Another approach to address solubility issue was taken in U.S. Pat. No. 5,820,962 (1998) by introducing substituted trivalent metal as the central atom of phthalocyanine. Due to the bulkiness of the proposed structure, the compound dissolved well in polar solvents and the resulting discs showed good reflectivity. However, polar solvent inherited the hydrophilic character and inevitably led to the difficulties in absorbing moistures during recycling. Consequently, it resulted in quality inconsistency and even deterioration of the disc performances.
In addition to solubility, dye sensitivity is another critical factor for recording media, particularly to enable high-speed recording and fast access to the recorded information. Addition of the so-called “pit edge control agent” was proposed in U.S. Pat. No. 5,492,744 (1996) and JP-A-798887 to improve deviation and jitter properties. Ferrocenes and its derivatives (e.g. benzoylferrocene and n-butylferrocene) blended with substituted phthalocyanine at certain ratios were suggested. Pit formations were reported to be largely improved but material utilization had become an issue in the real-life practices. Since optical dyes account for considerable ratio in recordable disc cost structure, dyes (and dye solutions) have been designed and synthesized to be recycled. Phthalocyanine exhibits better solubility in the designated solvent (ethylcyclohexane, in this case) than ferrocene does. Consequently, the blended-in pit edge control agent tends to precipitate out during spin coating and recycling, resulting in the undesired concentration changes in the recycled dye solutions. The yield (productivity) is therefore inferior to those with single dyes. A minor modification was seen in U.S. Pat. No. 5,789,138 (1998), in which phthalocyanine was blended with (or dissolved in) melted additive (e.g. benzimidazole) so that coordination occurred from the additive to the center metal of phthalocyanine. The thus-obtained dyestuff would exhibit better intermolecular associations to obtain desired film pattern. However, the trade-off between the limited coordination chemistry and the corresponding dye performance made it difficult to optimize disc performances.
Halogenation on phthalocyanine was also reported to improve sensitivity. U.S. Pat. No. 5,646,273 (1997) claimed OPC (optimal power calibration, or “optimal recording power”) was effectively improved by halogenation on the alkyl and/or alkoxy substituents to phthalocyanine. Halogenation directly on the phenyl ring of phthalocyanine, on the other hand, was also proposed in U.S. Pat. No. 6,087,492 (2000). However, these resulting discs still showed insufficient sensitivity and unsatisfactory controls in the formation of information pits. Nevertheless, precise reaction control in the degree of halogenation was difficult. The resulting compound was inevitably a mixture containing various numbers of halogen atoms, leading to unstable dye quality and inconsistent disc properties.
In U.S. Pat. No. 6,087,492 (2000), substituted phthalocyanine with divalent metal as the central atom was formylated, further reduced, followed by esterification. Without pit edge control agent in the structure or blended in the formula as described in U.S. Pat. No. 5,492,744 (1996), the resulting dye did not render satisfactory properties. Improvement was made in U.S. Pat. No. 6,399,768 B1 (2002) and U.S. Pat. No. 6,790,593 B2 (2004) by chemically bonding ferrocene to phthalocyanine through ester linkage. These metallocenyl phthalocyanines were halogenated (mainly brominated) at various degrees, depending on the central metal atoms. It was claimed that the resulting dyes exhibited good optical sensitivity and solubility to solvents such as di-butyl ether (DBE) and ethylcyclohexane (ECH). Although these dyes demonstrated satisfactory recording properties at high recording speeds, it did not show equally good performances at low recording rates. Particularly at 1× recording, the discs presented imprecise pit lengths and accordingly, less-than-satisfactory deviation properties.
To address the aforementioned disadvantages derived from the conventional techniques, this invention provides novel optical dyes with unique chemical linkages between the substituted or unsubsituted ferrocenes and phthalocyanine derivatives. Discs made of these novel optical dyes show excellent performances at 1× through 52× recordings.